An escapement answering the definition exposed in the paragraph above is known under the denomination of Swiss escapement lever where each of the pallet-stones receives alternatively an impulse to launch a balance with which said escapement is fitted, the escapement wheel set and the pallet being maintained at rest while this balance achieves its so-called supplementary arc freely. The Swiss lever escapement is the escapement most commonly used in the watch industry. In the majority of the cases its pallet is equipped with pallet-stones substantially arranged at equal distance from a line connecting the axis of the escapement wheel set to the staff on which the pallet is articulated, this wheel set comprising a single escapement wheel.
A Swiss lever escapement answering the definition exposed in the first paragraph of this description is described in the patent document CH-570 644. This document proposes an escapement with high output. It recalls that the escapement in a watch is used to transmit the energy of the driving spring to the balance which acts as the regulator of the running of the watch. However the running precision, and in particular the sensitivity of the running of a watch to disturbances, is closely related to the energy which can be accumulated in the sprung-balance system. This energy depends on several factors, as inter alia, the output of the gear train and the output of the escapement. The total output of the escapement can be divided into an escapement wheel-pallet output and a pallet-balance output.
In modern watches, the output of the gear train and the pallet balance output is more or less optimized. That is however not the case of the escapement wheel-pallet output and working on its improvement corresponds to an increase of the quality of the running of the watch.
According to the cited document, the improvement of the escapement wheel-pallet output can be done by increasing the energy which is provided to the balance at the time of the impulse given to the pallet-stones of the pallet by the teeth of the escapement wheel. The same document shows that this increase in energy is quite real if the pallet-stone describes a trajectory which makes, at the point of contact with the tooth of the escapement wheel, an angle as small as possible with the trajectory of said tooth. This can be obtained while moving away the entry pallet-stone and by bringing closer the exit pallet-stone, which causes a maximum bringing together of the directions of displacement of said pallet-stones and the direction of the displacement of the teeth of the escapement wheel.
FIG. 1 shows by way of example an escapement drawn according to principles of the prior art where the theoretical instantaneous outputs are clearly improved compared to a conventional escapement. One recognizes in particular the driving wheel set 4 in the shape of a single escapement wheel and the receiving wheel set 9 in the shape of a pallet, both turning around their respective arbour and staff 5 and 3. The escapement wheel 4 carries teeth 8 which transmit driving energy to the entry and exit pallet-stones 6 and 7 of pallet 2, which, via its fork 9, transmits it to the balance, respectively to impulse-pin 10 of roller 11 of the balance. According to the prior invention, the entry 6 and exit 7 pallet-stones embrace a number of teeth 8 at least equal to the fifth of the total number of teeth carried by the escapement wheel 4. Moreover, the entry pallet-stone 6 embraces, starting from a line 12 connecting arbour 5 of wheel 4, at least one tooth 8 more than the exit pallet-stone 7 embraces, starting from said line.